Zona Pellucida, Diagram Function, Structure, and Formation
Zona pellucida is a glycoprotein layer that surrounds the plasma membrane of mammalian oocytes and is required for fertilization. Zona Pellucida NEET Biology Notes are provided in the article below.
Krati Saraswat5 Aug, 2024
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Zona Pellucida: The zona pellucida is a glycoprotein layer that encases mammalian oocytes (egg cells). It is located between the plasma membrane of the oocyte and the surrounding corona radiata. The zona pellucida is crucial for successful fertilization and triggers the acrosome reaction, which allows sperm to penetrate the secondary oocyte.
The term "zona pellucida" comes from Latin and means "transparent zone." It refers to the specialized layer surrounding the mammalian egg, also known as the egg coat. This layer is vital for the growth of the oocyte and plays a key role in fertilization. The zona pellucida is an extracellular matrix that protects the egg cell and facilitates sperm entry. It is encased by the corona radiata, a layer of cells that support the egg after it is released from the ovary. For detailed information on the zona pellucida's function, structure, and formation, see the article below.Also Check:
What is Zona Pellucida?
The zona pellucida (ZP) is a layer of material that forms between the cell membrane of the oocyte (egg) and the surrounding granulosa cells during the early development of an ovarian follicle. This layer is about 5 to 10 μm thick and is made up of four main glycoproteins: ZP1, ZP2, ZP3, and ZP4. These glycoproteins, produced by the oocyte, create a complex mesh. Granulosa cells send out thin projections called filopodia that go through the zona pellucida and connect with tiny extensions on the oocyte. This connection allows the cells to communicate through gap junctions. The zona pellucida is essential for fertilization. ZP3 and ZP4 act as receptors for sperm. When sperm come in contact with these receptors, it starts a reaction that allows the sperm to break through the zona pellucida and reach the oocyte. Once a sperm fuses with the oocyte, a reaction called the cortical reaction happens. This releases enzymes that change the zona pellucida so that no more sperm can bind to it. This prevents multiple sperm from fertilizing the oocyte, ensuring that only one sperm fertilizes the egg.Also Check:
Zona Pellucida Diagram
The following is a diagram of zona pellucida:
Zona Pellucida Struture
The zona pellucida is an essential structure surrounding the oocyte (egg cell) in mammals and is crucial for fertilization. Here is a detailed overview of its structure and function: Composition:-
Proteins:
The zona pellucida is mainly made up of glycoproteins, including ZP1, ZP2, ZP3, and occasionally ZP4. These glycoproteins are secreted by the oocyte and create a thick extracellular matrix around it.
- ZP1: This protein helps maintain the stability of the zona pellucida.
- ZP2: It is vital for sperm binding and contributes to the formation and maintenance of the zona pellucida.
- ZP3: This glycoprotein is crucial for the initial attachment of sperm to the zona pellucida and is often considered the primary sperm receptor.
- ZP4: Found in some species, it also aids in the structural integrity of the zona pellucida and supports sperm binding.
- Thickness: The zona pellucida varies in thickness but is generally about 15-20 micrometers thick in humans. It is thicker than the oocyte’s cell membrane.
- Organization: The zona pellucida is a translucent, elastic membrane surrounding the oocyte. It consists of a network of glycoproteins forming a gel-like matrix.
| NEET Biology Notes | |||
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| Embryo | Funaria | Selaginella | Malvaceae |
| Polyembryony | Apomixis | Pinus | Pollen Grains |
| Chara | Volvox | Endosperm | Equisetum |
Zona Pellucida Formation
The zona pellucida begins to form in mammalian embryos long before fertilization occurs. In fact, it is present in oocytes, immature egg cells in the ovaries. These oocytes have the potential to mature into egg cells that travel through the fallopian tubes to the uterus during ovulation. Each oocyte is already encased in a zona pellucida, a transparent, protective layer. This ensures that if the oocyte develops into an egg and is subsequently fertilized, it will have optimal conditions for successful development. It is crucial to understand that the zona pellucida forms while the cell is still an oocyte, well before it matures into an egg and has the chance to be fertilized. The human zona pellucida is created through the combination of four glycoproteins. Glycoproteins are proteins with attached carbohydrate molecules. The oocyte, in collaboration with the ovarian follicles, synthesizes these glycoproteins. As these molecules come together, they form the zona pellucida matrix, a protective sphere surrounding the egg cell.| NEET Biology Notes | ||
|---|---|---|
| Plasma Membrane Structure | Pre-Fertilisation | Economic Importance of Algae |
| Slime moulds | hypotonic solution | Post fertilisation |
How does the Zona Pellucida Work?
The zona pellucida plays several essential roles in protecting egg cells and assisting in successful fertilization by sperm. Firstly, the zona pellucida helps sperm cells attach to the egg cell by having specific receptors composed of glycoproteins. These receptors are uniquely shaped to bind only to sperm from the same species as the egg. As a result, sperm from other species cannot attach to the zona pellucida and thus cannot reach the egg. Once a sperm cell from the correct species binds to a receptor on the zona pellucida, a series of steps is initiated that allows the sperm to penetrate the zona pellucida and fertilize the egg. This interaction also triggers changes in the zona pellucida that prevent additional sperm from entering the egg. Subsequently, the zona pellucida undergoes further alterations that make it even more difficult for other sperm cells to attach or pass through.Zona Pellucida Function
The zona pellucida is integral to successful fertilization and performs several key functions:- Role : The zona pellucida is situated between the oocytes (egg cells) and the follicular cells. It acts as a connection between these two cell types during oogenesis, the process of egg development.
- Protection : The zona pellucida provides protection to oocytes and embryos throughout their developmental stages, ensuring their safety and integrity.
- Acrosome Reaction Initiation : The zona pellucida initiates the acrosome reaction, which is critical for sperm cells to successfully bind to and penetrate the egg. This reaction is triggered when sperm cells bind to a specific glycoprotein in the zona pellucida known as ZP3.
- Prevention of Polyspermy : It helps prevent polyspermy, a condition where multiple sperm cells fertilize a single egg, which could lead to developmental issues.
- Species-Specific Fertilization : The zona pellucida ensures that fertilization occurs only between sperm and egg of the same species, thus preventing cross-breeding between different species.
- Regulation of Sperm-Egg Interaction : It regulates the interaction between free-swimming sperm and ovulated eggs or secondary oocytes, facilitating proper fertilization.
- Immunocontraceptive Target : T he zona pellucida contains sperm binding proteins (ZP1, ZP2, ZP3, and ZP4) that are potential targets for immunocontraceptive methods, which aim to prevent pregnancy by interfering with these interactions.
- Infertility Due to Absence : Females who lack the zona pellucida layer surrounding their eggs are infertile, as this layer is crucial for the normal reproductive process.
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|---|---|---|---|
| Rhizopus | Fibrous root | Marchantia | Actinomycetes |
| Nostoc | Spirogyra | Ribosomes | Liverworts |
Q 1. What is the function of the zona pellucida membrane?
Ans. The zona pellucida is a protective extracellular matrix surrounding the plasma membrane of an egg cell. Its primary functions are to safeguard the egg and to play a crucial role in the fertilization process. This membrane is encased by the corona radiata, a layer of cells that support the egg when it is released from the ovary.
Q 2. What secretes the zona pellucida?
Ans. The zona pellucida is secreted by the oocytes (immature egg cells) and the ovarian follicles (fluid-filled sacs that contain these oocytes).
Q 3. Where is the zona pellucida found?
Ans. The zona pellucida is present in every oocyte (immature egg cell) and forms a spherical layer around each egg cell.
Q 4. What does the zona pellucida do to sperm?
Ans. The zona pellucida regulates sperm entry into the egg cell. It initially allows only sperm from the same species to attach. Once a single sperm has attached, the zona pellucida undergoes changes that facilitate fertilization by that sperm. Subsequently, its structure changes to prevent additional sperm from entering.
Q 5. Who produces the zona pellucida?
Ans. The zona pellucida is produced by the oocyte and consists of four glycoproteins: ZP1, ZP2, ZP3, and ZP4. These glycoproteins create a mesh-like structure around the oocyte, with ZP1 cross-linking the repeating ZP2 and ZP3 units.
Q 6. Who digests the zona pellucida?
Ans. Acrosin is the enzyme responsible for dissolving the zona pellucida, which is essential for the sperm to fuse with the egg cell. Therefore, the correct answer is 'Acrosin'.
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